Several investigators have reported that morphine treatment causes an acute elevation of serum prolactin levels in the rat. There is at least one report that morphine has a similar action in humans. In addition, women addicted to heroin are subject to menstrual irregularities and infertility, and such disorders may be related to elevated serum prolactin levels. Few studies have focused on the effects of narcotic antagonist analgesics on prolactin secretion. One such agent, pentazocine, causes an elevation of serum prolactin in women. In the rat, however, the narcotic antagnoist analgesics buprenorphine and cyclazocine have been reported to cause a decrease in serum prolactin. In our preliminary studies, nalorphine causes a dose-related decrease in serum prolactin, wheras cyclazocine had no effect. In the proposed study, the effects of prolactin secretion of a broad spectrum of opiate receptor agonists, antagonists, and agents with a mixture of agonist and antagonist activity, will be evaluated systematically. Agents with selective actions on three putative types of opiate receptors will also be evaluated. It is important that the effects of narcotic antagonist analgesics on prolactin secretion be resolved, since the absence of a hyperprolactinemic action might encourage the use of these agents in place of more addictive analgesics.